a) Field of the Invention
The present invention relates to a surface light source device using a light conducting member.
b) Description of the Prior Art
FIG. 1 shows a construction of a conventional surface light source device using a light conducting member. This device comprises a light conducting member 11, a linear light source 12 arranged in the vicinity of one of edge surface 11a (light incident surface) of the light conducting member, a diffusion plate 13 located on or adjacent to a light emitting surface 11b of the light conducting member 11 and a reflective surface 14 disposed on or adjacent to an opposite side surface 11c of the light emitting surface of the light conducting member 11. In this surface light source device, a light emitted from the light source 12 is made incident upon the light incident surface 11a of the light conducting member 11, and then transmitted inside of the light conducting member toward an opposite edge surface of the light incident surface 11a. The light directed into the light conducting member 11 in this manner is emitted from the light emitting surface 11b while being transmitted inside of the light conducting member 11. Then the light emitted from the light emitting surface 11b passes through the diffusing plate 13 to form a diffused light, which is available for use as a surface light source.
In such a surface light source device, a pattern of fine diffusing portion 20 is formed on the opposite side surface 11c of the light emitting surface 11b of the light conducting member 11 in order to increase a light emitting efficient of the light which is made incident into the light conducting member 11, and to get a uniform brightness distribution on the diffusing plate 13, which allows to have a uniform and bright surface light source. The pattern is formed by printing or something like that with the aid of painting material having a light diffusing function.
Since such a surface light source device is used, for example, as a backlight for a liquid crystal display unit, it is required to produce a brighter light. However, the above-explained conventional surface light source device cannot provide an enough brightness for such an application. Therefore, various means are suggested on such type of the surface light source devices to increase the brightness of the emitting light.
As typical surface light source devices to which means for increasing the brightness is applied, there are disclosed in Japanese Patent Kokai Publication No. 3-189679 and Japanese utility Kokai Publication No. 3-31782. In the conventional surface light source devices disclosed in these publications, a large number of pyramid-shaped convex or concave portions are formed on the opposite side surface of the light emitting surface of the light conducting member in order to increase the brightness. Further, these convex or concave portions are arranged to be varied according to the area chosen to produce a uniform brightness distribution.
The conventional devices, however, have a problem in that the amount of the emitting light of the device is larger in the area closer to the light source 12 and decreases as farther from the light source 12, so that a uni-form surface light source device cannot be obtained. Particularly, in case that the light source 12 is provided only one of the edge surfaces of the light conducting member and the distance between the edge surface of the light source side and the opposite side edge surface is long, the difference of the amount of the emitting light between the area closer to the light source and the area far from the light source becomes extremely large.
In order to solve this problem, there is suggested another surface light source device in which a light reflecting member is provided on the edge surface opposite to the light incident edge surface of the light conducting member. However, in this device, the amount of the emitting light is decreased even in the area which is only a little far from the light reflective member. Thus no good effect can be obtained by the light reflective member and it is difficult to get a uniform brightness distribution even from the newly suggested light source device.
Further, in the above-explained conventional devices, the light reflected on the surfaces of the convex or concave portions formed on the opposite side surface of the light conducting member is directly emitted from the light emitting surface. Therefore, the amount of the emitting light emitted from the portions of the light emitting surface corresponding to the convex or concave portions on the opposite side surface becomes large, so that there is a difference in brightness between the portions corresponding to the convex or concave portions and the other portions, which causes an unevenness of brightness.
In order to solve this problem, it is suggested to arrange as a coarse surface on an inclined surface of the conical or polygonal pyramid-shaped concave portions. In such devices, the light arrived at the inclined surfaces of the concave portions is diffused by the coarse surface and therefore, it is possible to prevent that the brightness becomes partially large due to that the emitting light emitted from the portions corresponding to the concave portions is larger in comparison with the light emitted from the other portions.
However, such devices are not in practice use, be cause it is difficult to make coarse surfaces on the inclined surfaces of the fine concave portions, each of whose diameter is extremely small, i.e. about 0.1-1.0 mm. Particularly, it is much more difficult to control the coarse ratio of the inclined surfaces of the small concave portions to be uniform and obtain the firstly demanded coarse ratio. Therefore, even if the coarse surfaces can be arranged inclined surface of the concave portions, the coarse ratio would not become uniform and as demanded, and thus it would be difficult to obtain a uniform emitting light from the light conducting member.